1. Field of the Invention
This invention relates to an optical pickup device for a recordable disc, such as a magneto-optical disc, and a dichroic mirror employed in such optical pickup device. More particularly, it relates to an optical pickup device for a recordable disc employing two laser light beams of different wavelengths, and a dichroic mirror employed in such optical pickup device for separating the light beams with different wavelengths.
2. Description of Related Art
In a tape recorder or the like, among a variety of signal recording apparatus, which is employed for professional applications in, for example, recording studios, it becomes necessary to perform pre-reading to effect synchronous recording or post-reading to effect monitoring of recording. With conventional tape recorders for professional applications, a playback head, a recording head and a second playback head are placed in this order, or a recording head, a playback head and a recording head are arranged in this order in order to enable such pre-reading or post-reading. On the other hand, in a recording apparatus for a recordable disc, such as a magneto-optical disc, it has been contemplated to use two pickup devices in an optical system associated with one of the disc sides, with one of the pickup devices taking charge of signal recording and the other pickup device taking charge of signal reproduction. However, since the use of the two pickup devices complicates system implementation, it has also been contemplated to make use of two laser light beams with different wavelengths, which are synthesized by a dichroic mirror to produce a sole laser light beam which is irradiated by means of an object lens on the disc as plural light spots or so-called multi-spots, with the reflected light being separated into laser light beams of the different wavelengths by means of the dichroic mirror or an interference filter to effect pre-reading or post-reading.
Although it would be highly effective to form such multi-spots on the disc by means of the sole object lens from the laser light beams of the two wavelengths, it may be feared that, if the light beams of the two different wavelengths cannot be separated positively from each other, the laser light beams will leak into each other at the photodetector elements, so that satisfactory signals cannot be obtained. On the other hand, with the use of the magneto-optical disc as the recordable disc, since the small Kerr rotation angle is taken out as an electrical signal, satisfactory signals cannot be obtained as a result of de-phasing of the P-polarized light component or the S-polarized light component.
If the wavelengths of 780 nm and 830 nm are selected as specific examples of the two wavelengths of the laser beams, and detection of photomagnetic signals is performed at the side of the return beam of the wavelength of 780 nm on the basis of the difference between the P-polarized light component and the S-polarized light component, a dichroic mirror used for separating the light beams of the respective wavelengths has transmittance characteristics with respect to the wavelength as shown, for example, in FIG. 10 and phase difference characteristics between the P-polarized light component and the S-polarized light component with respect to the wavelength as shown, for example, in FIG. 11 That is, since photomagnetic signal detection is performed by the return laser beam of the wavelength of 780 nm, the dichroic mirror should have the phase difference characteristics shown in FIG. 11, in which the phase difference becomes zero in the vicinity of 780 nm.
Meanwhile, if a semiconductor laser with a wavelength of 780 nm or 830 nm is employed, a separation bandwidth of the order of 30 nm is necessitated in order to take account of fluctuations of the laser oscillation wavelengths, shifting in wavelengths due to temperature changes or widening of the wavelengths due to superimposed high harmonics. It is however extremely difficult to separate the two wavelengths completely while maintaining the zero phase difference in the vicinity of the wavelength of 780 nm. For this reason, it has been customary to provide an interference filter having wavelength-transmittance characteristics as, for example, shown in FIG. 12 in a return beam path of the laser light having a wavelength of 780 nm and to provide another interference filter having wavelength-transmittance characteristics as, for example, shown in FIG. 13 in a return beam path of the laser light having a wavelength of 830 nm to improve wavelength separation characteristics. However, the return light of the laser light of one of the wavelengths reaching the interference filter provided in the return beam path of the laser light of the other wavelength for elimination thereby is diminished in light volume because of the incomplete wavelength separation by the dichroic mirror. Although it may be contemplated to provide a large gap between the two wavelengths for a more complete wavelength separation, the object lens needs to be of higher performance, thus leading to elevated costs.